1. Field of Invention
The present invention relates to a metal halide lamp, and more specifically to a horizontally lighted metal halide lamp with a ceramic discharge tube with high efficacy.
2. Description of Related Arts
A metal halide lamp with a ceramic discharge tube has been widely used. For an ordinary metal halide lamp with a ceramic discharge tube, a discharge cavity is formed in the ceramic discharge tube, the discharge cavity is filled with inert gas (such as xenon Xe) and ionizable salt, a pair of electrodes placed in the discharge cavity are located at two ends of the ceramic discharge tube respectively, a certain interval exists between tips of the two electrodes so as to form a discharge path there-between. Currently, no matter whether the ceramic discharge tube of the ordinary ceramic metal halide lamp is of a one-body type, two-body type, three-body type, or even five-body type in early time, the geometric shape of the appearance of the ceramic tube body has a common characteristic: the ceramic discharge tube is always formed of three parts, that is, a fat cylinder or ellipsoid (as shown in FIG. 2) of a middle discharge part (as shown in FIG. 1) and two co-axial thin cylinders at two sides.
To increase the light emitting efficiency of the metal halide lamp with a ceramic discharge tube, currently the metal halide lamp with a ceramic discharge tube with high efficacy is designed with a structure as shown in FIG. 3, in which the ceramic discharge tube usually adopts a slender ceramic tube body, and such slender ceramic tube body is characterized in that the maximum inner diameter D1 in the center is less than the distance L1 between the electrodes. Although the ceramic discharge tube adopting such mechanism may achieve high efficacy certain disadvantages also exist. Firstly, such slender ceramic tube body is small in the inner diameter, and is large in the tube wall load, and when the slender ceramic tube body is horizontally lighted, arc bending makes the temperature of the central part at the upper end of the tube wall excessively high, thereby causing deformation of the tube wall of the ceramic tube body, or even fracture, so as to seriously influence product reliability and safety.
In addition to the disadvantage that the excessively high temperature in the center of the upper end of the tube wall causes deformation or fracture of the tube wall of the ceramic tube body, another prominent problem of the ceramic discharge tube in such slender design is: the tube body is slender, the “cold spot” is located at the two ends, which is far away from the center of the arc when the tube body is horizontally lighted, and the position of the “cold spot” is not fixed, so the filler in the ceramic discharge tube has no fixed condensation point, thereby causing the color temperature to be unstable, so that the color temperatures variation among lamps is great, and the color temperature of a single lamp drifts significantly with time, so as to seriously influence the illumination quality of the lamp.
Furthermore, currently, the filler in the usually used metal halide lamp is generally metal halide series containing rare earth metal such as DyI3, HoI3, and TmI3. These metal halides may enable a lamp to have excellent color rendering properties, but these metal halides have very strong corrosivity at high temperature, so these metal halides are not appropriate candidates to be used in a high efficacy ceramic metal halide lamp efficacy.